Fix flaky vertex pass-through spend test by polling instead of fixed sleep

The test used a fixed 40s sleep before checking spend logs, but async
spend logging in CI sometimes takes longer to flush. Replace with a
polling loop (10s interval, 120s max) that exits early on success.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/pass_through_tests/test_vertex_ai.py

@@ -109,17 +109,25 @@ async def test_basic_vertex_ai_pass_through_with_spendlog():
 
     print("response", response)
 
-    await asyncio.sleep(40)
-    spend_after = await call_spend_logs_endpoint()
-    print("spend_after", spend_after)
+    # Poll for spend update instead of fixed sleep - spend logging is async/batched
+    max_wait = 120  # total seconds to wait
+    poll_interval = 10  # seconds between checks
+    elapsed = 0
+    spend_after = spend_before
+    while elapsed < max_wait:
+        await asyncio.sleep(poll_interval)
+        elapsed += poll_interval
+        spend_after = await call_spend_logs_endpoint() or 0.0
+        print(f"spend_after (elapsed={elapsed}s)", spend_after)
+        if spend_after > spend_before:
+            break
+
     assert (
         spend_after > spend_before
-    ), "Spend should be greater than before. spend_before: {}, spend_after: {}".format(
-        spend_before, spend_after
+    ), "Spend should be greater than before after {}s. spend_before: {}, spend_after: {}".format(
+        elapsed, spend_before, spend_after
     )
 
-    pass
-
 
 @pytest.mark.asyncio()
 @pytest.mark.skip(reason="skip flaky test - vertex pass through streaming is flaky")